1. Field of the Invention
The present invention relates to a servo circuit for use with an air pressure actuator.
2. Description of the Related Art
As an air pressure actuator, there has been one which was suggested by the inventors of the present invention and is shown in FIG. 1. Referring to FIG. 1, such an air pressure actuator 10 comprises a guide shaft 11 extending in one axial direction with both ends thereof fixed on a pair of support members 15, and a slider 12 movable along the guide shaft 11. In fact, the slider 12 is a cylindrical hollow body so formed that it can cover up a part of the guide shaft 11, corresponding to a predetermined length thereof. In this way, an internal space is formed between the inner surface of the slider 12 and the outer periphery surface of the guide shaft 11. Practically, such an internal space is used as a pressure chamber. In more detail, such an internal space serving as a pressure chamber has been divided (in its axial direction) into two cylinder chambers 16a and 16b by virtue of a partition wall 13 fixed on the guide shaft 11. The slider 12 is slidable over the outer periphery surface of the partition wall 13. At both ends of the slider 12 and with the cylinder chambers 16a, 16b are connected intake/exhaust systems 17a, 17b for introducing a compressed air into the cylinder chambers and for discharging the same therefrom. The intake/exhaust systems 17a, 17b are respectively equipped with servo valves 18a, 18b so as to form desired servo systems. These servo valves 18a, 18b are all connected to a compressed air supply source 100.
For example, when the servo valve 18a is actuated to supply a compressed air, while the servo value 18b is actuated to discharge an amount of used air into the surrounding atmosphere, the partition wall 13 will be caused to act as a pressure receiving plate and the slider 12 will be moved to the left direction shown in FIG. 1. In this way, by controlling the opening degree of the servo valves 18a and 18b, it is allowed to move the slider 12 to any desired position along the guide shaft 11.
The air pressure actuator 10 is provided with a position detector, thereby forming a position feed back control system.
However, as will be described later in the present specification, in an arrangement where each of the intake/exhaust system is connected with only one servo valve (as shown in FIG. 1), the speed resolving power of the air pressure actuator has been found deteriorated.
Accordingly, it Is an object of the present invention to improve the response and the speed control performance of a servo circuit which is for use with an air pressure actuator.
The servo circuit according to the present invention can be used in an air pressure actuator which has a partition wall provided on one of a slider and a guide shaft to form pressure chambers between the outer surface of the guide shaft and the internal surface of the slider, and to define these pressure chambers into two cylinder chambers arranged side by side in the axial direction.
According to an aspect of the present invention, intake/exhaust systems are connected with the two cylinder chambers for supplying a compressed air into the cylinder chambers or discharging the same therefrom. A plurality of servo valves having different maximum opening degrees are respectively connected with the intake/exhaust systems. The servo circuit further includes a position detector for detecting the position of the slider. Moreover, the servo circuit includes a control device for receiving the detection results of the position detector and a position instruction value, so as to perform a selection among a plurality of servo valves in accordance with an acceleration or deceleration zone and a constant speed zone of the slider, and to control an opening degree of a selected servo valve.